1. Technical Field
This invention relates generally to post-sintering densification of powder metal preforms and more particularly to the densification of inner peripheral surfaces of openings provided in powder metal preforms.
2. Related Prior Art
Powder metal components have long offered a low cost alternative for many equivalent wrought components. Powder metal forming eliminates the high cost of machining associated with wrought components. However, in applications where high strength components are required, such as transmission components and hard bearing surfaces for example, traditional powder metal components generally have not exhibited sufficient strength and dimensional tolerances to serve as low cost alternatives to the high strength wrought components in some cases.
A technique commonly used to densify inner peripheral surfaces of openings in powder metal components is roll forming. U.S. Pat. No. 4,059,879 discloses such a roll forming method. According to the disclosed method, a powder metal bearing ring is engaged on its inner surface by a forming roll which, when rotated with an applied force to the preform ring, acts to compress and densify the inner surface to provide a hardened inner race. Additional strength can be imparted to powder metal components by inducing a buildup of compressive stresses in addition to densification. However, the roll forming technique described above does not provide the necessary impact force to achieve a high level of compression stress buildup in the powder metal preform as would be desirable. In addition, special provision must be made to support the preform for rotation without encumbering the inner surface, and the forming tool must be sufficiently small to fit within the opening while being capable of exerting sufficient compaction force to densify the material, both of which add complexity and cost to the manufacture of powder metal components.
It would thus be desirable in the manufacture of powder metal components to provide a process whereby the inner peripheral walls of openings can be densified more efficiently to yield powder metal components comparable in strength and dimensional tolerance to that of wrought components.